Add module field for ESM distribution

[FND]

As far as I can tell, jsnext:main is deprecated in favor of module (see https://github.com/rollup/rollup/wiki/pkg.module and rollup/rollup-plugin-node-resolve#209).

Just to be safe, I decided to duplicate rather than replace the jsnext:main field - not sure whether that's actually necessary though.

[btd]

Well, first of all thank you for PR. I can merge it if you need for example for your profile, but actually this PR is meaningless. I do not support shouldjs modules outside of should.js.

[FND]

Anything's fine by me - just for context:

I do not support shouldjs modules outside of should.js

A user of @faucet-pipeline (which uses Rollup under the hood) wanted to include shouldjs in a single JavaScript bundle (not sure why or whether that's ever a good idea). import should from "should" resulted in Rollup complaining:

'hasOwnProperty' is not exported by node_modules/should-util/cjs/should-util.js"

So I guess you never know how modules are gonna be used in the wild. ¯\_(ツ)_/¯

Having said that, we've since fixed support for jsnext:main in faucet, so this isn't an immediate issue anymore. Proposing an upstream patch just seemed like the right thing to do, being a good citizen and all.

[btd]

Ah, ok. Will merge then